1. Field of the Invention
The present invention relates to a semiconductor device and a process for producing same.
2. Description of the Related Art
Conventionally, when an SOI (silicon-on-insulator) device formed on an insulator is combined or united with a device formed on a silicon substrate, such as a power MOS transistor, etc., the resultant increase in current capacity causes the substrate to act as a drain, and thereby, the device formed on the insulator occasionally becomes unstable due to a fluctuation of the drain voltage occurring upon operation of the power MOS transistor.
A solution to this problem was proposed in Japanese Unexamined Patent Publication (Kokai) No. 62-213272. In the proposed arrangement as shown in FIG. 1, on a LOCOS-oxidized film as a first insulating layer 201, an electroconductive thin film 204 as a shield is formed by the same process steps as the forming of the gate electrode 203 of a vertical MOS transistor 202, a second insulating layer 205 is formed on the electroconductive thin film 204, and a semiconductor elements 206 and 207 are then formed on the second insulating layer 205 so that the thus formed SOI device is shielded from a fluctuation of the substrate electric potential due to the operation of a vertical MOS transistor.
In this arrangement, however, the SOI device portion naturally has a greater height than the vertical MOS transistor region as seen from FIG. 1 and the thus formed uneven or stepped surface causes a disconnection of conductor layers at the steps, particularly in highly integrated devices having a multilayered conductor structure.
Moreover, such an arrangement is not advantageous as an IC circuit in that an SOI device formed by depositing a polycrystalline silicon (hereinafter simply referred to as "polysilicon") on the second insulating layer cannot provide a high device performance obtained when an SOI device is formed in a monocrystalline silicon.